Generally in portable digital devices such as a mobile phone or a PDA (Personal Digital Assistant) phone having a mobile phone function, a keypad device having a limited number of keys arranged by a specific method is used as a Hangeul input device. A conventional keypad is configured to have 10 keys to each of which a different number is assigned, and 2 extra function keys.
In the Hangeul input method using the keypad having a limited number of keys, in the case that a plurality of consonants or vowels are assigned to a key, a method of assigning consonant, vowel, or function keys for Hangeul to the keypad, or a method of selecting a plurality of consonants or vowels, plays an important role in Hangeul input speed, the number of key operations required for the input, and convenience of the input.
As a typical Hangeul input method using a keypad developed up to the present time, there exists an input method (shown in FIG. 1A) using a so-called “Cheonjiin” system in which “Cheon” (), “Ji” () and “In” () respectively indicate heaven, earth, and human according to the principle of the Hangeul invention. The Hangeul input method according to the Cheonjiin system does not directly assign vowels of Hangeul to a keypad but assigns ,   each to a separate key. This Hangeul input method utilizes a keypad configuration for inputting vowels by using combinations of the 3 keys, wherein consonants are assigned to other keys.
As another typical Hangeul input method, there exists a method (shown in FIG. 1B) in which basic consonants are assigned to individual keys, basic vowels are assigned to other keys, and extended consonants or combined vowels are formed by using function keys such as an additional stroke key or double consonant key. Because there are more consonants than vowels in the basic phonemes of Hangeul and the vowels may easily be converted by using a function key such as an additional stroke key, a keypad used in such a Hangeul input method adopts a configuration in which the number of consonant-assigned keys is greater than the number of vowel-assigned keys.
Additionally, the conventional keypad has a configuration in which function keys such as an additional stroke key are disposed at a generally used location without considering the flow lines of a user's key operation with fingers on the keypad. For example, a key * and a key # are used as function keys in the case of a keypad having 12 keys.